1. Field of the Invention
The present invention relates to a GaN epitaxial substrate and a method of fabricating the same and to a nitride semiconductor device and a method of fabrication the same.
2. Related Background Art
Gallium nitride (GaN) is attractive material of substrates for light emitting devices. For example, Japanese Patent Application Laid-Open No. 2001-102307 publication discloses the formation of a GaN substrate. In this formation of GaN substrates, a GaN thick layer is grown by the lateral growth method on a semiconductor single-crystal substrate such as a gallium arsenide (GaAs) substrate. Then, the GaAs substrate is removed from the GaN thick layer and one or more GaN substrates are formed from the GaN thick layer.
Prior to manufacturing semiconductor devices thereon, an epitaxial layer is grown on such a GaN substrate to form a GaN epitaxial substrate. As disclosed in Japanese Patent Application Laid-Open No. 2000-340509 publication, a homoepitaxial layer of GaN, which has the same composition as the substrate, has been grown thereon as the epitaxial layer because researchers thought that the growth of the homoepitaxial layer can reduce the number of defects such as threading dislocations. Particularly, the homoepitaxial layer was used for GaN based semiconductor laser diodes because threading dislocations in the semiconductor layers of the laser diode affect the lifetime thereof.